1. Field of the Invention
The invention relates to an electrical cable adapted for high-voltage applications. The electrical cable can be used with fixed apparatuses which are either permanently installed or stay at a given location, such as office equipment, home appliances, etc.. Such apparatuses may use or produce high voltages, in which case some parts of them can generate high-voltage noise. The present invention more particularly concerns electrical cables for the high-voltage circuits used in those parts susceptible of generating high-voltage noise.
2. Description of Background Information
Known electrical cables for high-voltage circuits may be classified into two categories. The first category includes a cable system in which copper-conductor cables are generally used, but in which downstream steps employ cables which contain a ferrite core in order to suppress noise (prior art 1). The second category includes a cable system that uses reinforcing cables made of an aramide fiber, a glass fiber or the like, on the surface of which conductive carbon is baked and adhered. With this type of cable, noise is suppressed by increasing the impedance of the carbon portion of the conductive cables (prior art 2).
It is also known that improved high-voltage breakdown resistance can be achieved by twisting together a plurality of conductive threads 1 to form a cable suitable for high-voltage circuits (FIG. 1). With this cable, the surface of the twisted conductive threads 1 is made uniformly smooth, so that electrical voltage is prevented from concentrating on particular points. To this end, the twisted conductive threads 1 are coated with an electrically conductive resin 2 through an extrusion process, and are then provided with an insulating coating 3 (prior art 3).
With this prior art 3, a material having a good high-voltage breakdown resistance and a good extrudability, such as low-density polyethylene (LDPE) or cross-linked LDPE, may be used as the insulating coating 3. However, pure polyethylene resins are inflammable. As it is now required that office or home appliances must be nonflammable, flame retarders are usually added to these resins to meet this requirement.
With a cable for high-voltage circuits which includes a ferrite core portion (prior art 1), it is difficult to suppress noise over a broad frequency spectrum. Therefore, additional means have to be adopted for effective noise suppression. However, these additional means involve extra costs, due to the supplementary manufacturing steps they require.
When a conductive cable is prepared by adhering carbon around a reinforcing thread through a baking process (prior art 2), the impedance may be set to a high level in order to remove high-voltage noise. However, the resulting conductive cable has a structure which does not form inductance elements, and therefore noise cannot be suppressed efficiently.
With prior art 3, the electrically conductive resin 2 will become thermally deteriorated after a long-term use, and fine cracks may form on the surface thereof. Then, the voltage will become concentrated in those cracks. When a high voltage is charged in this state, dielectric breakdowns may occur, and the conductive threads 1 can then no longer serve as a high-voltage cable.
In addition, the end portions of the electrical cable must be prepared for high-voltage circuits by connecting metal terminals thereto. In the case of prior art 3, the connections established during this preparation process can sometimes be made through the electrically conductive resin 2, which causes impedance fluctuations. The impedance may also vary after prolonged use, owing to the deterioration of electrically conductive resin 2. Moreover, the grip for holding the terminals may be weakened, with the high-voltage resistance subsequently being deteriorated.
Further yet, when a low-density polyethylene is used, as is the case with prior art 3, the electrical cable deforms at high temperatures. This may lead to some cable characteristics, such as its behavior during the so-called "high-voltage cutting-through test", to deviate from the standards adopted by Underwriter's Laboratories Inc. (UL Standards) in force in the United States. In such a case, a flame retarder can be added to make the cable more fireproof However, such an additive lowers the voltage breakdown resistance of the cable. A solution would be to maintain the breakdown resistance by making the insulating coating thicker. However, such a measure would be at the expense of the plasticity of the cable, the resulting electrical cable for high-voltage circuits then becoming less flexible.